1. Field of the Invention
The present invention relates to a speed control mechanism and a mechanical timepiece having the same.
2. Description of the Prior Art
In a speed control mechanism of a mechanical timepiece, when a balance with hairspring is constituted by a balance stem and accessories thereof (hairspring, hairspring bead, oscillating seat and the like) and a balance wheel, the speed control mechanism of the mechanical timepiece includes a balance with hairspring, a hairspring, a stud structure and a regulator structure. The hairspring in a mode of a spiral spring is fixed to the hairspring bead press-fitted to the balance stem at an inner side end portion in a radius direction thereof, fixed to the stud at an outer side end portion in the radius direction, and an effective length thereof is adjusted by a regulator having a hairspring rod or a hairspring receive brought into contact with the hairspring at a vicinity of the outer side end portion in the radius direction. The balance with hairspring is reciprocally pivoted centering on the balance stem grossly at a period rectified by the effective length of the hairspring under control of an escapement including an escape wheel & pinion and a pallet fork. There is also well known a structure enabling to roughly adjust and finely adjust a position in a peripheral direction at which the hairspring rod or the hairspring receive is brought into contact with the hairspring (for example, JP-A-48-19262). Further, it is also known that a magnitude of an oscillating angle (rotating angle) of the balance wheel effects an influence on the period of reciprocally pivoting the balance with hairspring and effects an influence on a rate of a timepiece.
Various kinds of improvements have been carried out over many years in order to optimize structures and shapes of respective portions constituting a speed control mechanism and minimize friction or the like of relatively moving portions to minimize loss in reciprocally pi voting a balance with hairspring. The improvements include forming a hairspring by a ferromagnetic material to make a temperature coefficient of Young's modulus positive in order to cancel a thermal expansion of the hairspring by a temperature change of Young's modulus to minimize a temperature dependency in a reciprocally pivoting period of the balance with hairspring.
On the other hand, a frequency adjusting structure of a regulator structure and a regulator fine adjustment structure is seen immediately in removing a case back of a timepiece and therefore, in a background art, the frequency adjusting structure is formed by a material whose major component is iron such as carbon steel in order to meet various requests of clearly polishing a structure in consideration of an outlook thereof, to make the structure as thin as possible to minimize an increase in a thickness of a movement and necessitating a strength for a material to be able to adjust a pivoting position only when a large force is operated thereto. The material is a (ferro) magnetic material.
Further, also with regard to a frequency settling structure such as a stud support, there is a case in which the structure is formed by a material whose major component is iron such as carbon steel by reason similar to that of the frequency adjusting structure and the material is a (ferro)magnetic material.
On the other hand, according to JP-B-44-15925, an escape wheel is covered by a large portion of a plate-like extended portion provided at a pallet fork bridge. A material having a high permeability is used for the pallet fork bridge. Thereby, although the escape wheel cannot be moved by 55 oersted in an ordinary constitution, the escape wheel can be moved up to about 80 oersted.
The inventors have noticed that there is a possibility that a magnetic field effects an influence on a reciprocating characteristic of a balance with hairspring when the inventors have analyzed a characteristic of oscillation of the balance with hairspring, and when the inventors have carried out a test, to the inventor's surprise, the inventors have found that magnetization of a regulator structure effects an influence which is difficult to be disregarded on an oscillating angle of the balance with hairspring (experimentally confirmed).
The invention has been carried out in view of the above-described point and it is an object thereof to provide a speed control mechanism capable of minimizing a reduction in an oscillating angle of a balance with hairspring and a mechanical timepiece having the same.